PROJECT SUMMARY/ABSTRACT The immune system is a decentralized network of diverse white blood cells that is distributed across the entire organism. Contributing to its complexity, the immune system has developed mechanisms by which it remembers its prior experiences and is influenced by them. Recently, strategies to activate the immune system of cancer patients to recognize and eliminate tumor cells have resulted in significant clinical progress. These immunotherapies attempt to initiate new, potent immune responses in individuals with tumor burden. While inspiring, the majority of cancer patients do not respond to these therapies, especially those with adenocarcinomas, which are the most prevalent forms of cancer. Thus, there is a pressing need to understand why these therapies often fail and use this knowledge to develop more effective strategies. One of the major barriers to accomplishing this goal is our poor understanding of how the state of the immune system differs in cancer patients compared to healthy individuals. While it is well appreciated that tumors actively evade immune responses, we still do not understand how this context shapes the ability of the immune system to initiate and execute new responses. This gap in our knowledge has partially resulted from insufficient methods to measure and interpret the immune state. However, we have recently developed experimental and computational methods capable of modeling the state of the immune system under any condition, identifying how its behavior differs from the healthy setting. Therefore, in this proposal, we plan to apply these tools to provide a comprehensive assessment of the immune state in tumor-bearing individuals. We will begin by understanding how the immune system adapts its organization and behavior across the body during the development and spread of a tumor. We will subsequently assess how immune cells that have experienced tumor burden change their propensity to respond to new stimuli, including molecules that have potential as immunotherapies. Lastly, we will reveal how the immune system responds differently to new challenges in healthy versus tumor-bearing individuals. The result of these studies will be a comprehensive understanding of how the experience of a tumor alters the behavior of the immune system. This knowledge will be essential for the development of new immunotherapies for cancer by revealing which pathways of immune activation remain functional or become enhanced in tumor-bearing individuals. Moreover, this proposal will reveal mechanisms by which the immune system incorporates its prior knowledge when making decisions with broad implications for all immunological processes including those in infection, autoimmunity and transplantation.